1. Field of the Invention
The present invention relates to novel cyclic organosilicon compounds and a method for using thereof as a component of catalysts for olefins polymerization. In olefins polymerization or copolymerization, in particular, in propylene polymerization or copolymerization, Ziegler-Natta catalyst systems comprising the cyclic organosilicon compounds as the external electron donor component exhibit dramatically-improved hydrogen response, and therefore can be used to prepare polymer having high melt-flowability and high isotacticity at high yield.
2. Description of the Related Art
Ziegler-Natta catalysts for olefins polymerization or copolymerization are well known in the art. These catalyst systems are typically composed of a transition metal-containing active component, generally comprising magnesium, titanium and halogen as main ingredients; and a co-catalyst component, usually an organo-aluminum compound. In the utilization of Ziegler-Natta catalysts for polymerization of alpha-olefins in which isotacticity is a possibility, electron-donating compounds are widely used (1) as an internal electron donor in the solid Ziegler-Natta catalyst component and/or (2) as an external electron donor to be used in conjunction with the solid Ziegler-Natta catalyst component and the co-catalyst component.
It is known in the art that external electron donors act as stereoselective control agents to improve isotacticity, i.e., stereoregularity of the resulted polymer products, by selectively poisoning or converting the active sites of non-stereoregularity present on the surface of a solid catalyst. Also, the catalyst activity, polymerization kinetics, molecular weight, molecular weight distribution, and melt flowability of the resulting polymer depend on the choice of external electron donors. Therefore, in order to improve the olefin polymerization process and the polymer properties, there is always a desire to develop new external electron donors with better combined features.
In propylene polymerization or copolymerization, organo-silicon compounds are commonly used as external electron donors in the Ziegler-Natta catalyst system. Typically, such external electron donors have silicon as the central atom containing at least one Si—OR, Si—OCOR, or Si—NR2 bonds, where R is commonly an alkyl, alkenyl, or aryl group with 1-20 carbon atoms. Such compounds are described in U.S. Pat. Nos. 4,472,524; 4,473,660; 4,560,671; 4,581,342; 4,657,882; 5,106,807; 5,407,883; 5,684,173; 6,228,961; 6,362,124; 6,552,136; 6,689,849; 7,009,015; 7,399,812; 7,619,049; and 7,790,819.
Currently the global market has an increasing demand in polypropylene having high melt flowability (high MFR grade polypropylene) to reduce cycle time and to achieve down-gauging while maintaining acceptable impact strength and stiffness. High MFR grade polypropylene is commonly achieved by adding peroxide to the polymer, but such obtained polypropylene usually has odor issues and reduced physical properties. So, production of reactor-grade high MFR polypropylene becomes necessary to avoid these issues.
However, when using the main catalyst components and the external electron donors disclosed in the prior art, polypropylene with a melt flow rate of 50 g/10 minutes or more is impractical due to the insufficient reactivity of molecular weight controlling agents, such as hydrogen. Owning to the designed pressure limit of polymerization reactors, the possible amount of hydrogen feed is practically limited, thus it is impossible to increase the hydrogen pressure to the extent that allows the production of polypropylene having a high melt flow rate, based on prior art methods.
To overcome this production limit, novel external electron donors have been developed in the art to improve the hydrogen response of the catalyst system. Japanese patent publication No. Showa 57-63311 discloses a Ziegler-Natta catalyst system using phenyltriethoxysilane as the external electron donor which, however, has a toxicity issue due to phenyl groups released from the silane donor after polymerization. JP-A8-143620 proposes a method for propylene polymerization using a dialkoxysilane having two aliphatic amino substitutions as the external electron donor, but polymerization activity and polymer stereoregularity are not satisfactory when producing high MFR grade polypropylene. U.S. Patent Publication No. 2005/5202958 A1 discloses aminotrialkoxysilanes (e.g. U donor) as external electron donors in the Zieglar-Natta catalyst system for propylene polymerization, but the molecular weight distribution is narrow on production of polymers with high MFR. U.S. Pat. No. 7,244,794 teaches a method for producing propylene polymer having a high melt-flowability by using a mixed external donor system, in which a secondary electron donor, dicyclopentyldimethoxysilane (DCPDMS) has to be added to the primary alkyltrialkoxysilane donor to boost up the catalyst activity. It also fails to describe any molecular weight distribution.